Standard Test Method for Field Measurement of Sound Power Level by the Two-Surface Method

SCOPE
1.1 This test method covers the field, or  in situ measurement of sound power level by the two-surface method. The test method is designed to minimize the effects of reverberant conditions, directivity of the noise source under consideration, and the effects of ambient noise from other nearby equipment operating at the same time.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

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ASTM E1124-97 - Standard Test Method for Field Measurement of Sound Power Level by the Two-Surface Method
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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: E 1124 – 97
Standard Test Method for
Field Measurement of Sound Power Level by the Two-
Surface Method
This standard is issued under the fixed designation E 1124; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 4. Summary of Test Method
1.1 This test method covers the field, or in situ measurement 4.1 The average one-third or full octave band sound pres-
of sound power level by the two-surface method. The test sure levels are measured over two different conformal surfaces
method is designed to minimize the effects of reverberant which envelop the equipment. These conformal surfaces
conditions, directivity of the noise source under consideration, should be selected to consist of rectangular, cylindrical, and
and the effects of ambient noise from other nearby equipment hemispherical constituent surfaces so that the surface areas
operating at the same time. may be easily calculated. From the difference between the two
1.2 This standard does not purport to address all of the average sound pressure levels taken at each surface and from
safety concerns, if any, associated with its use. It is the the areas of the surfaces the sound power level may be
responsibility of the user of this standard to establish appro- calculated. The calculation accounts for both the effect of the
priate safety and health practices and determine the applica- reverberant field and the noise of other equipment. It is
bility of regulatory limitations prior to use. permissible to define conformal surfaces that completely en-
velope the source, yet only measure over a portion of the
2. Referenced Documents
conformal surface due to restrictions from process connections
2.1 ASTM Standards:
or accessibility.
C 634 Terminology Relating to Environmental Acoustics
5. Significance and Use
2.2 ANSI Standard:
S1.4 Specification for Sound Level Meters 5.1 The function and operation of equipment in the field
often preclude the measurement of the free-field sound pres-
3. Terminology
sure levels of a single piece of equipment in the absence of
3.1 Definitions—For definitions of terms used in this test
interfering sound from other equipment operating at the same
method, refer to Terminology C 634. time. The two-surface method will provide, in most cases, a
3.2 Definitions of Terms Specific to This Standard:
reliable estimate of the normal sound power levels of a
3.2.1 conformal surface—the locus of points which lie at a specimen operating in an adverse environment.
fixed distance from the reference surface of a piece of
5.2 This test method is intended for use in the field in the
equipment. Two conformal surfaces are used in this test presence of what is normally regarded as interfering back-
method. These are surfaces over which the measuring micro-
ground noise. This test method is based upon the work of
,
5 6 7
phones are swept. They are located at two different distances Hubner and Diehl, but differs from all other current sound
from the equipment. Fig. 1 shows a typical arrangement of
power measurement procedures by requiring simultaneous
these surfaces for a generalized piece of equipment. measurement at both conformal surfaces and by resolving
3.2.2 constituent surface area—a portion of the conformal
time-averaged sound pressure levels at both surfaces to within
surface. 0.1 dB. These two features, simultaneous recording and 0.1-dB
This test method is under the jurisdiction of ASTM Committee E-33 on
Environmental Acousticsand is the direct responsibility of Subcommittee E33.08 on Hubner, G., “Analysis of Errors in Measuring Machine Noise Under Free Field
Mechanical and Electrical System Noise. Conditions,” Journal of the Acoustical Society of America, Vol 54, No. 4, 1973, pp.
Current edition approved Sept. 10, 1997. Published June 1998. Originally 967–977.
published as E 1124 – 86. Last previous edition E 1124 – 92. Hubner, G., “Qualification Procedures for Free Field Conditions for Sound
Annual Book of ASTM Standards, Vol 04.06. Power Determination of Sound Sources and Methods for the Determination of the
Available from American National Standards Institute, 11 W. 42nd St., 13th Appropriate Environmental Correction,” Journal of the Acoustical Society of
Floor, New York, NY 10036. America, Vol 61, No. 2, 1977, pp. 456–464.
4 7
Terminology C 634 – 85 was the edition used during the development of this Diehl, G. M., Machinery Acoustics, J. Wiley and Sons, New York, NY, 1973,
test method. pp. 97–103.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E1124–97
FIG. 1 Configuration of Conformal Surfaces, General Case
resolution, enable source sound power to be calculated when 6. Operating Conditions
the direct sound field of the source is actually lower in level
6.1 Whenever possible, equipment under test must be oper-
than the ambient noise.
ating in a mode acceptable to all parties involved in the test.
5.3 The use of this test method is expected to be primarily
Otherwise operating conditions must at least be monitored in
for the relative assessment of the sound power from similar order that the test results are properly qualified in terms of
sources or for the prediction of sound levels in a plant based running speeds, flow rate, production rate, etc.
upon measurements of similar sources in another plant. This
7. Apparatus
test method is believed to be capable of yielding a reasonably
good estimate of absolute power level with proper care of 7.1 Due to the amount of data which must be gathered and
application and full conformance to the provisions of this processed, the following are considered to be the minimum
procedure. equipment necessary to meet the requirements of this test
procedure.
5.4 The two-surface method is applicable only when the two
7.1.1 Microphones, that are matched in terms of frequency
measurement surfaces can be physically selected to produce
and pressure response. Begin by calibrating each data channel,
positive values of the difference in average sound pressure
using the same calibrator on each channel. Connect both
level. That is, the inner surface sound pressure level minus the
microphone channels to the cables, connectors, amplifiers, and
outer surface sound pressure level must be at least + 0.1 dB.
recorder to be used in data gathering. Then arrange the
This limitation applies to each frequency band and each
microphones side by side in the presence of broad band
constituent surface area investigated. Only the frequency band
ambient noise and record for 60 s on both channels. The
in which a zero or negative difference occurs is considered
differences in the averaged sound pressure levels in each
invalid and usually adjacent bands will be valid. In practice
frequency band are calibration corrections which may be
only rarely will all three one-third octave bands of a given
applied to either channel prior to any calculation.
octave yield invalid data at all constituent areas. Therefore less
7.1.2 Magnetic Tape Recorder, two-channel instrumentation
than complete results are permissible when one-third octave
grade having a frequency response of 61 dB over the
analysis is used and full octave results are reported.
frequency range of interest.
5.5 The two-surface method may not produce results when
7.1.2.1 It is recognized that even high-quality Amplitude
testing some very large machines in very reverberant rooms or
Modulation (AM) tape recorders cannot maintain channel-to-
in rooms having a volume less than about 20 times the space
channel frequency response within 0.1 dB. It is believed,
enclosed by an envelope around the larger dimensions of the
however, that the requirement for determining the corrections
machine. In such cases, the sound pressure level close to the
in 7.1.1 based on 60-s average readings sufficiently compen-
machine may not decrease in any regular way with increasing
sates for expected instabilities, channel-to-channel. If digital
distance from a machine surface, making it impossible to select frequency modulation (FM) or pulse code modulation (PCM)
two measurement surfaces producing positive differences of
tape recorders are used, the procedure of 7.1.1 should still be
sound pressure level. used.
E1124–97
NOTE 1—The frequency response and accuracy of the acoustical
level between the two surfaces for the frequency range of
instruments are different from the interchannel resolution of the tape
interest. As stated in Section 5, merely a 0.1 dB difference in
recorder. Both the frequency response discussed in 7.1.2 and the accuracy
average sound pressure levels constitutes a measurable drop.
of the acoustical calibrators are distinctly different from the 0.1-dB
However, the surfaces should be chosen so as to maximize the
resolution discussed in 5.2.
difference since the overall accuracy of the estimated sound
7.1.3 Microphone Mounting Fixture—A suggested fixture is
power levels will be thereby improved. Obviously, the closer
shown in Fig. 2.
the inner surface is to the equipment, the easier it will be to
7.1.4 Spectrum Analyzer, real-time one-third or full octave,
obtain a large positive difference, but possible near-field effects
having a resolution of 0.1 dB with a digital display or printing
dictate an inner surface farther from the equipment. Such
capabilities.
near-field effects cannot be quantified by this test method nor
NOTE 2—Real-time analyzers having a resolution of 0.25 dB may also
can their effect on the calculated power levels be determined,
be used. However, because of the requirement for a positive sound level
so that this procedure can only suggest that the inner surface
difference, as discussed in 5.4, these analyzers may yield less complete
microphone be always at least 0.15 m, and for larger machines
results compared with what could be obtained with an analyzer with better
resolution. In addition, the precision of the results will be reduced if only at least 0.3 m, from the equipment surface thereby avoiding
differences greater than 0.25 dB can be obtained.
most such effects.
7.2 Optional equipment may include:
8.1.2 If the locations of the two conformal surfaces are too
7.2.1 Programmable Calculator or Desktop Computer.
close together, measurable differences in average sound pres-
7.2.2 Data Processing, direct from output of real-time
sure levels will be difficult to obtain. On the other hand, no
analyzer.
advantage is gained by using progressively larger outer sur-
faces once the outer surface microphone is in the fully
8. Procedure
reverberant field since the sound level, and therefore the
8.1 Selection of Measurement Surfaces:
differential, will be constant. No clear optimum ratio between
8.1.1 Conduct a preliminary survey of the sound field to
these two surface areas can be prescribed for all equipment. As
estimate the two optimum conformal measurement surfaces
a guide, however, experience has shown that an area ratio of
that will yield a measurable drop in average sound pressure
FIG. 2 Example of Suggested Measurement System
E1124–97
about 1.4 to 2.0, between the outer and inner surfaces, is a 8.1.6 No optimum distances from the equipment surface to
reasonable range that may be used in most cases. either conformal surface can be prescribed for all equipment.
8.1.3 Select simple geometric shapes for conformal sur-
However, for sources whose smallest dimension is 1 m, it is
faces. Fig. 1 shows an example of a generalized situation. In
recommended the inner surface distance be at least 0.2 m. Also,
Fig. 1, even though the equipment itself can be approximated
for sources whose smallest dimension is 3 m, it is recom-
by rectangular or cylindrical surfaces which just enclose the
mended the outer surface distance be less than 2 m.
equipment, the reference surface is chosen so that the two
8.2 Data Acquisition:
conformal measurement surfaces are convex. It may be helpful
8.2.1 Obtain simultaneous measurements of the sound pres-
to imagine the major equipment reference surfaces to be
sure level at the two microphone positions along a line normal,
defined by a membrane stretched over the equipment after the
that is perpendicular to, the inner conformal surface. See 7.1.3
removal of minor projections, gages, tubes, and cables not
for a suggested microphone mounting fixture. Determine the
expected to be noise sources themselves. Ideally, the sound
average sound pressure level over each constituent surface area
intensity vector would be normal to both measurement surfaces
using a continuous uniform microphone sweep as indicated in
at all points. Although this cannot be determined using this test
Fig. 4.
method it may be helpful if the surveyor will attempt to
visualize the expected sound field and so might adjust the 8.2.2 If the inner and outer measurement surfaces are
selection of conformal surfaces accordingly. subdivided into smaller constituent areas for the survey, the
8.1.4 It is permissible to subdivide the conformal surfaces
average sound pressure levels over the entire inner and outer
into several constitutent surface areas for ease of data collec-
conformal surfaces are determined by summing the values
tion or because of inaccessibility. Any number of constituent
obtained for the respective constituent areas, as shown in 9.3.
surface areas may be used to cover the conformal surface.
8.2.3 The microphone sweeping speed shall be sufficiently
Since the conformal surfaces will be measured simultaneously
slow, continuous, and uniform that when the data are continu-
with the inner and outer microphones, care should be taken that
ously recorded a representative average sound pressure level is
the constituent surface area boundaries define related regions
obtained for each constituent area swept by the microphone(s).
on the inner and outer surfaces. These constituent surface areas
A reasonable averaging period is usually between 30 and 60 s
will not necessarily be composed of geometrically similar inner
for each constituent area. A reasonable sweeping speed is
and outer surfaces because of the usually complex shape of the
usually about 0.5 m/s.
equipment sources themselves.
8.2.4 Fig. 5 illustrates an alternate data collection technique
8.1.5 Fig. 3 is an example of the application of these
in which a large number of uniform constituent surface areas
guidelines for the selection of measurement surfaces. A large
are measured by moving the m
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